The invention relates to a method of coding digital image data, particularly for the purpose of transmission over channels having a limited transmission capacity, while employing transform coding which converts the video image or partial regions thereof by means of a transformation rule into another representation. The representation of the image data is effected by a number of coefficients corresponding to the number of pixels in the original image region. Such coefficients, however, have only a considerably lower correlation in contrast to the pixels of the video image which may have large correlations. That is, the coefficients are substantially uncorrelated while the pixels may be greatly correlated as in regions of the image where the gray scale value is substantially uniform. The independence of the coefficients allows for a selection of a subquantity of coefficients, which may be considerably smaller than the number of original pixels. The selection can be based on the coefficient amplitudes, e.g. all coefficients whose amplitudes exceed a predetermined magnitude. The selected coefficients are quantized, which reduces the amount of data further. Using the selected and quantized coefficients produces, after reversal of the transformation rule, an approximate reconstruction of the original image region.
It is known to process digital image data for transmission over channels having a limited transmission capacity by subjecting them to transform coding, e.g. a DCT (discrete cosine transform) or a Walsh-Hadamard transform as is described, for example in U.S. Pat. No. 4,805,017; and Esprit '86, Results and Achievements, Commission of the European Communities, Directorate General XIII, Telecommunications, Information, Industries & Innovation, Elsevier Science Publishers B.V., 1987 (North Holland), pages 413-422. All prior art methods of transform coding have in common that the images or partial image regions (segments) to be subjected to transformation have a rectangular or often even square shape. The images are divided into blocks by a regular grid and these blocks are transform coded separately. The condition that only rectangular partial image regions can be transformed has the result that, on the one hand, interference patterns in the form of the subdivision predetermined by the block grid, so-called blocking effects, occur and, on the other hand, uniform regions are unnecessarily subdivided and thus the attainable data compression is limited.
According to Digital Image Processing, William K. Pratt, published by John Wiley & Sons, New York, N.Y., U.S.A., pages 232-278, the image produced by the transformation can be represented by the weighted sum of a set of basis functions. The basis functions are here fixed for the entire image (see, for example, page 245, FIG. 10.3-2, Cosine Basis Functions) and are given by the type of transformation. The basis functions may, for example be in the form of polynomials or trigonometric functions.